Disposable pump cassettes are frequently employed to infuse medicinal fluids into a patient. As described in U.S. Pat. Nos. 4,818,186 and 5,000,664, one type of disposable cassette includes a plastic housing having a front and rear portion, between which an elastomeric membrane is encapsulated. The housing has a plurality of ports through which rod-like actuators of a pump drive mechanism interact with the elastomeric membrane to control fluid flow through channels formed within the cassette housing. A pump plunger on the drive unit displaces the membrane to pressurize liquid trapped in a pumping chamber formed between the membrane and the back of the housing.
Through another port in the housing of the cassette, a pressure sensor rod impinges against the elastomeric membrane to sense the pressure of the fluid in a fluid passage that is adjacent an outlet port of the cassette. The pressure sensor rod is coupled to a strain gage pressure transducer that produces a signal indicative of the pressure of fluid at the outlet port. A microcontroller in the pump drive controls the pump plunger, actuator rods and pressure sensor to effect a desired rate of delivery of medicinal fluids to the patient, and in some units, is capable of selecting between a plurality of different sources by opening an appropriate selector valve adjacent one of a plurality of inlet ports in the cassette.
Significant leakage through the valves in the pump cassette can create a potentially harmful variation from the programmed quantity of medication delivered to a patient, or in the case of a leaking selector valve, may allow a medicinal fluid to freely flow through the pump cassette when infusion of the fluid into the patient is not desired. The pump assembly detects leakage of the valves by conducting a valve test upon the valves when the cassette is initially loaded into the pump drive and thereafter, each time that the pump is energized. Valve leakage is determined as a function of the signal produced by the pressure sensor. An alarm sounds if a leak is detected during the valve leakage test to alert an operator of the problem.
The pump assembly is connected to a patient by a tube set and catheter. Vibrations in the tubing occur during normal operation whenever the patient moves and/or the operator bumps or otherwise displaces the tubing. These motion induced vibrations create hydrodynamic pressure noise. The hydrodynamic pressure noise may be sufficiently great in amplitude to interfere with the valve leakage test. If the noise level causes a false alarm indicating valve leakage, fluid flow to the patient is interrupted while the operator tries to correct the problem. Alternatively, the noise level may obscure a true leakage problem. In view of the potential harm to the patient should false alarms frequently occur, there is clearly a need for minimizing the likelihood of hydrodynamic pressure noise interfering with the valve leakage test.
Prior to implementing the present invention, no attempt was made to prevent hydrodynamic noise from interfering with the valve leakage test. The obvious method for dealing with this type of problem would be to filter any hydrodynamic pressure noise from the pressure transducer signal using a fast Fourier series transform or an electronic filter. Valve leakage would then be determined as a function of the filtered signal. However, this conventional method would require a series of complex calculations by the microcontroller, resulting in reduced efficiency due to the processing load, and possibly requiring a more sophisticated, expensive microcontroller. The present invention provides a much simpler and more elegant method for minimizing hydrodynamic pressure noise interference with a valve leakage test that obviates the need for complex calculations and a sophisticated microcontroller.